The efficiency of processes which use light to make physical or chemical changes in material can be affected by the amount and/or spatial distribution of light, which in turn can be affected by the size, configuration and positioning of the light source, reflectors and object to be treated. For some processes, it is difficult to measure the amount of light which impinges on a surface because of limited access for placing or installing instrumentation. Typical of these processes are those in which a reflector or light source is extremely close to the object being irradiated. In some processes, light is impinging on a surface from more than one direction and in order to ensure accuracy, light measurements must be made immediately adjacent to that surface. In other processes, problems arise due to the intensity of the radiation which prevents or limits the use of conventional compact radiometers.